1. Field of the Invention
The present invention relates to an optical pickup device that uses an near-field light to record data to and/or reproduce data from an optical recording medium, an optical recording/reproducing apparatus including such an optical pickup device, and a gap-control method used in such an optical pickup device.
2. Description of the Related Art
In recent years, for attaining high recording density and high resolution in optical recording media such as optical disks and optical memory cards, a recording/reproducing system using near-field light (also called evanesent wave) generated at an interface between substances when the distance between them is shorter than a certain distance has been used. In such a recording/reproducing system using near-field light, a gap between a device applying near-field light, such as a lens, and the surface of an optical recording medium may need to be within an extremely small range of typically about ½ to ⅕ of the wavelength of light to be used in recording and reproducing process.
A focusing optical system that generates near-field light may include an objective lens with a high numerical aperture, such as an aspheric lens, and a solid immersion lens (SIL) between the objective lens and an optical recording medium. Such a system using the SIL may need to keep the distance (gap) between the SIL and the surface of the optical recording medium, such as an optical disk, at a distance allowing the generation of near-field light, about ½ to ⅕ or less of the wavelength of light as described above. In this case, furthermore, the posture of the SIL should be controlled to follow the wobbling of the surface of the optical recording medium or disk-wobbling of the disk-shaped optical recording medium. In order to control the posture of the SIL, for example, a gap-control method for keeping a desired gap constant by detecting the gap based on a total-reflection return-light quantity has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2001-76358).
This control method utilizes the fact that the size of the gap and the total-reflection return-light quantity are in a proportional relationship at a distance that allows the generation of near-field light. In other words, in the method, the total-reflection return-light quantity is used to have a gap-error signal. Also, a feedback-servo loop is formed by stabilizing a servo loop system with a phase compensation filter to hold the gap constant.
For example, 20 nm is selected as a desired value for keeping a distance allowing the generation of near-field light, 5 nm is an allowable deviation, and 40 μm is an allowable surface-wobbling level. Here, a disk-shaped optical recording medium is used and the rotational speed thereof is set to 3,000 rpm (revolutions per minute). In this case, the necessary bandwidth may be 8 kHz or more. In actual, however, disturbance caused by the rotation of the disk strongly outputs a rotationally synchronized component, so that it is difficult to control the gap with high accuracy even if a bandwidth of 8 kHz or more is ensured.